As generally known, ESD protection circuits are essential for protecting integrated circuits from high-voltages caused by transient discharge. Such high-voltage transients can be induced by specific ESD events, i.e., through contact from one's fingertips or even during the manufacturing process of integrated circuits. For instance, a person can generate very high voltages of a few hundred and up to a few thousand Volts by simply moving over a carpet.
Moreover, there is an increasing tendency to apply exchangeable IC's in nowadays electronic systems. Thus, only a minimal number of IC's need to be replaced instead of the whole circuit board in order to upgrade, for example, microprocessors or memory cards. Therefore, adequate ESD protection for integrated circuits is essential in order to avoid damages.
Conventional ESD protection circuits are usually integrated between an IC input pin, also known as pad, and the IC itself, providing a current path that ensures that the voltage applied to the IC clearly stays below a specific critical limit.
However, conventional ESD protection circuits, whether internal on a chip or in an external device, are often not suitable for high-frequency (RF) applications, due to the adverse impact such ESD protection approaches have on the performance of a given high-frequency (RF) device. Also, standard circuits operating at low frequencies or at low speeds are relatively insensitive to the parasitic resistance, capacitance and inductance introduced by use of standard ESD protection circuits.
For example, FIG. 1 illustrates a conventional ESD protection circuit comprising two diodes used as ESD-elements (D1, D2), wherein the signal input pad of an integrated circuit is directly coupled to the ESD-elements (D1, D2). Although, the ESD-elements represent a parasitic capacitance that could affect the performance of the integrated circuit.
As a result, conventional ESD-elements can not be used at very high frequencies. For example, a parasitic capacitance of 100 fF would generate an impedance of a mere 20.7Ω at a frequency of 77 GHz.
FIG. 2 illustrates another conventional ESD protection circuit that does not use ESD-elements for critical high-frequency pads. Instead, a direct current path is provided between the pad and the supply voltage using, for example, an inductor or transmission line (L1), though, many applications prefer to avoid such a direct current path.